1. Field of the Invention
This invention relates to a hot-element chemical vapor deposition process to grow epitaxial films on a substrate.
2. Description of the Prior Art
Methods for the deposition of an epitaxial silicon film, for use in technical applications, have typically been performed at high temperatures. Conventional deposition temperatures usually exceed 1,000.degree. C., or involve a preparatory high temperature cycle in order to clean a single crystalline substrate prior to the deposition of an epitaxial silicon layer. For example in Kagata, U.S. Pat. No. 5,221,412, a method is disclosed which provides for the vapor phase epitaxial growth of a silicon film on a single crystal substrate using diluted disilane, wherein the process is carried out at a temperature of approximately 1,000.degree. C. This operating temperature serves to both clean the substrate and thermally decompose the source gas. At these high temperatures dopants in the substrate, upon which the epitaxial layer is deposited, often diffuse from the substrate into the epitaxial layer during evaporation and deposition from the gas phase. As a result, the thickness of the epitaxial layer must be greater than the diffusion migration distance of the dopants from the substrate. These dopant diffusion dynamics thus dictate the ultimate thickness of the desired epitaxial thin-layer because the layer cannot be thinner than the diffusion migration distance. Because thin-epitaxial-layers are useful in order to reduce the overall dimensions of high-performance integrated circuitry, it is desirable to develop a low-temperature method to rapidly grow epitaxial silicon. In addition, a rapid low temperature method to make thin-film epitaxial silicon would reduce manufacturing costs.
The prior art has disclosed several methods for the production of epitaxial films with efforts directed toward reducing the substrate temperature during the evaporation phase. For example Kobayashi, U.S. Pat. No. 5,495,823, has disclosed a method in which a single crystalline thin-film is formed on a semiconductor substrate at temperatures not higher than 800.degree. C. In this method, as the process temperature is slowly increased from approximately 350.degree. C. to 620.degree. C., silane gas is carried onto a single crystalline silicon substrate to form an amorphous silicon thin-film. At the same-time the amorphous silicon thin-film slowly begins to single crystallize. When the temperature reaches 540.degree. C. the growth rate of the single crystalline silicon film exceeds that of the amorphous silicon film. The single crystal silicon thin film is finally formed at 620.degree. Centrigrade.
Another method, used to grow epitaxial films at lower processing temperatures, is disclosed in U.S. Pat. No. 5,298,452, issued to Meyerson. There, a method and apparatus for depositing single crystal, epitaxial films of silicon on a plurality of substrates in a hot wall, isothermal deposition system is described. The deposition temperatures are less than 800.degree. C., and the operating pressures during deposition are such that non-equilibrium growth kinetics determine the deposition of the silicon films. An isothermal bath gas of silicon is produced allowing uniform deposition of epitaxial silicon films simultaneously on multiple substrates. This is a flow system in which means are provided for establishing an ultrahigh vacuum in the range of about 10.sup.-9 Torr prior to epitaxial deposition. While the Kobayashi and Meyerson methods are carried out at comparatively lower temperatures, the Kobayashi method provides a slow indirect method, and the Meyerson method is carried at an ultrahigh pressure.
The National Renewable Energy Laboratory has pioneered the use of a direct low-pressure hot-element assisted chemical vapor deposition process for growing a semiconducting grade .alpha.-Si:H from the decomposition products of silane. See. e.g. R. S. Crandall, et. al., Sol. Cells 30, 15 (1991); A. H. Mahan, et. al., J. Appl.Phys., 69, 6728 (1991). Moreover, in Mahan, U.S. Pat No. 5,397,737, a similar hot-element deposition apparatus has been used to generate a low-hydrogen-content high quality hydrogenated amorphous silicon film by passing a stream of silane gas over a high temperature tungsten element. The Mahan disclosure is incorporated herein by reference. However, while the Mahan process uses a hot-element chemical vapor deposition reactor to form amorphous silicon below 400.degree. C., this invention provides a new and significantly different use of the hot-element apparatus to rapidly grow an epitaxial layer, such as silicon, at low substrate tempperatures.